1. Field of the Invention
The present invention relates to a differential pinion, a metal mold for plastic working the differential pinion, and a method for plastic working with the above metal mold, and particularly to a differential pinion having an end relief section, a metal mold for plastic working the above differential pinion, and a method for plastic working with the above metal mold.
2. Description of the Prior Art
FIG. 12 to FIG. 15 show a conventional differential pinion 10 which is used in differential gears for automobiles and others. This differential pinion 10 is formed by having a tooth section 13 formed on the outer periphery of a gear body 11. And in FIG. 12, a pitch angle is indicated by .theta. and a distance from a cone vertex O to a cone distance large end face B by L. FIG. 13 shows a sectional view taken on line 13--13 of the tooth section 13 in FIG. 12.
Heretofore, the above differential pinion 10 has its tooth face 14 formed by cutting work (see FIG. 12) or plastic working (see FIG. 14).
Lately, an end relief section may be provided on the cone distance large end face of the tooth face which is formed by cutting or plastic working, to remove the contact of the above large end face and to reduce a noise. FIG. 16 and FIG. 17 show differential pinions which are provided with end relief sections E.
Heretofore, the end relief sections E have been formed by cutting work.
As shown in FIG. 14, the production of the differential pinion 10 by plastic working uses closed forging which does not produce an excess wall which protrudes out of the cone distance large end face B, which is required to be removed by processing at the completion of forming the pinion.
FIG. 20 shows a closed forging metal mold to be used for closed forging. In this metal mold, the lower part of a cope 15 and the upper part of a drag 17 are provided with a shaped section 19 for plastic working a differential pinion 16. The cope 15 has a punch 21 inserted therein, and the drag 17 a counter punch 23 therein.
In the above closed forging metal mold, the cope 15 and the drag 17 are preliminarily pressed, and with the cope 15 and the drag 17 securely held, the punch 21 and the counter punch 23 are inserted, thereby capable of producing the differential pinion 16 not having an excess wall protruding out of the cone distance large end face B which is required to be removed by later treatment.
But, in the conventional differential pinion 10 shown in FIG. 12 and FIG. 14, the tooth section 13 has a working face covering the cone distance large end face B against a mating gear, inducing a drawback of producing noise readily.
ln the differential pinion 10 which is provided with an end relief face 18 shown in FIG. 16, when the tooth face 14 is formed by cutting work and then the end relief face 18 is formed by cutting work, fiber flow is disrupted at the tooth face 14 as shown in FIG. 18. Particularly, there is a drawback that separation and other defects readily occur at boundary point Z between the end relief face 18 and the tooth face 14. But, when the end relief face 18 is formed by cutting work after producing the tooth face 14 by plastic working, fiber flow is also disrupted as shown in FIG. 19, causing a drawback such as separation at about the boundary point Z.
The closed forging metal mold shown in FIG. 20 has a mold separation surface at the cone distance large end face B, which is positioned higher than the horizontal separation surface (surface D in the figure), and therefore the metal mold has poor strength and rigidity, resulting in a partial elastic deformation thereof. As shown in FIG. 21, it is substantially impossible to prevent the production of burrs in a space 19 between the cope 15 and the drag 17 and remedy an insufficient Wall thickness within the space 19. As a result, the differential pinion produced has drawbacks that it has a tight tooth contact around the cone distance large end face B and readily makes noise.